As a method of developing an electrostatic image in electrophotography, various dry development methods have been employed. In such dry development methods, generally a toner is charged by friction with an iron powder, glass beads called as a carrier and electrically attracted and adhered to a photo conductor in an electrostatic latent image and transferred to paper and fixed by a heating roller to give a permanently visible image.
A widely employed method as a fixing method is a heating roller method involving passing a toner image of an object sheet to carry a toner image thereon through the surface of a thermal fixing roller having the surface made of a separable material to a toner in a pressuring and contacting state.
In this thermally fixing roller method, to improve the economic property such as saving power consumption and increase the copying speed, a toner fixable at lower temperature has been desired.
However, if the above-mentioned low temperature fixation property is to be improved, an offset phenomenon that a portion of the toner adheres to the thermal fixing roller surface and re-transferred to paper tends to occur easily or a blocking phenomenon that the toner coagulation is caused by heat which the resin particles receive in various ambient environments tends to occur easily.
With respect to a conventional polyester type toner, chemical crosslinking structure is formed in the polymer by copolymerization of the polyfunctional, generally tri- or higher-functional monomers to retain high temperature offset resistant property. However, such a method is inferior in luster owing to the unevenness of the printed surface caused by the fixing roll after fixation owing to the existence of components which are not dissolved in the polymer or has a limit in the low temperature fixation property.
To deal with such problems, Japanese Patent No. 2,988,703 proposes use of crystalline polyester resin containing 50% or more by mole of repeating units obtained from terephthalic acid and straight chain type alkylene glycol having 2 to 6 carbon atoms in the total monomer units used as a binder resin for a toner. However, according to the technique proposed, only the crystalline polyester resin is used, the temperature range for possible fixation is narrow and thus it is difficult to keep the high temperature offset resistance and blocking resistance without deteriorating the low temperature fixation property.
Japanese Patent No. 2,704,282 proposes use of non-crystalline polyester resin obtained by polymerization of polyvalent monomers with tri- or higher-valence, aromatic dicarboxylic acids, and aliphatic alcohol containing 50% or more by mole of branched aliphatic alcohol as a binder resin for a toner. However, there are problems that the low temperature fixation property is insufficient because of sole use of non-crystalline polyester resin.
Japanese Kokai Publication Hei-4-97366 and 4-313760 propose toners containing two types of polyesters with different softening points for toner resins, as toners excellent in the balance between the low temperature fixation property and the high temperature offset resistance. However, the compatibility of these two type polyesters is not so sufficient to tend to cause blocking of the polyester with a low softening point or filming because of adhesion of the polyester to the fixing roller and further, since the compatibility is insufficient, the transparency of the resin is low.
Japanese Kokoku Publication Hei-5-44032 proposes use of block copolymers of low melting point crystalline polyesters and high melting point crystalline polyesters as binder resin for a toner. However, there is a problem that according to this technique, the binder resin becomes an opaque whitish resin and no transparent resin can be obtained.
Further, if the toner is exposed to a temperature equal to or higher than the glass transition temperature of the resin for the toner, blocking phenomenon is caused easily and therefore, possibility of using polyester resin for a toner which scarcely causes the blocking phenomenon has been investigated. As polyester resin for a toner which scarcely causes the blocking phenomenon although having not so low temperature for low temperature fixation, Japanese Kokai Publication Hei-4-337741 describes that it is effective to use polyester resin compositions with specified compositions and also Japanese Kokai Publication Hei-10-36490 describes that it is effective to use polyester resin with specified compositions and having glass transition temperature in a range of 45 to 70° C. However, although the blocking phenomenon is suppressed at a normal temperature by these techniques, even in the case of using such resins for a toner, blocking phenomenon is caused any way if toners are exposed to a temperature around the glass transition temperature of the resins for a toner.
Japanese Kokai Publication Sho-56-65146 discloses a toner for electrophotography comprising a binder composed of 50 to 95% by weight of an non-crystalline polyester with a glass transition temperature 40° or higher, a softening point of 80 to 150° C. and a number average molecular weight 1,000 to 10,000 and 5 to 50% by weight of crystalline polyester with a glass transition temperature 0° C. or higher, a melting point 110 to 220° C., and a number average molecular weight 8,000 to 40,000. The toner is improved in the fine pulverization property, blocking resistance, and the property of separation to a heating roller by use of two types of polyesters as a binder. However, it is difficult to sufficiently satisfy the low temperature fixation property, high temperature offset resistance, and storage stability only by simply mixing and using two types of polyesters and hard to provide sufficient durability and image reproducibility.